1. Field of the Invention
The present invention relates to an image forming apparatus for forming an image on an image supporter with toner charged with static electricity, transferring the toner image on the image supporter and then fixing the image. More specifically, the present invention relates to an image forming apparatus including a charge removal means for applying electric charge to remove static electricity of a recording medium on which a toner image is transferred, and a means for giving an offset preventive electric field to rotating members for fixing.
2. Related Background Art
An image forming apparatus of the electrophotography system transfers a toner image electrostatically held on a photosensitive member onto a transfer medium, and then removes static electricity of the transfer medium and separates it from the photosensitive member.
As such a separation means for separating the transfer medium from the photosensitive member after the transfer process, a corona discharge device, a static electricity removal needle, and the like is used.
The separation means performs discharge to give electric charge with same pole as that of toner to the transfer medium so that electric charge with reverse pole applied to the transfer medium by a transfer charge device is removed.
As a device for fixing the unfixed toner image, those of the heat roller system consisting of a pair of rotating members are generally used because of high efficiency and so on.
One of the drawbacks of the heat roller system is the offset phenomenon, wherein some of toner adheres to the rotating member which gets in contact therewith during the fixing process and a defective image occurs. Among many factors of this phenomenon, one of the most important factors is the static electricity of the rotating members which acts on toner. Since the fixing roller is generally coated with fluorocarbon resin or the like in order to prevent toner from adhering to the fixing roller, the resin is frictionally electrified with different sign with respect to toner while a sheet goes through the rollers. Thus toner is electrostatically attracted to the fixing roller, or toner is repelled by the pressure roller, thereby causing the offset phenomenon.
Accordingly, a countermeasure has been made, wherein bias voltage is applied to the rotating member for fixing to prevent the offset phenomenon by positively utilizing electrostatic attraction or repulsion.
Application of bias voltage to the rotating member for fixing is a good countermeasure against the offset phenomenon, but an apparatus having a charger for carrying out separation after the transfer process has the following problems.
(1) The offset phenomenon cannot fully be avoided with some kinds of paper or in certain environments.
(2) The sheet sometimes winds around the rotating member and cannot be completely separated therefrom.
The above-mentioned phenomenon (1) is likely to happen when a lot of static electricity of the sheet is removed during the separation process, which is explained as follows. During the transfer process, the sheet and toner are charged with different signs from each other and are electrostatically bound to each other. But, if much static electricity is removed with same sign, little static electricity remains in the sheet, and sometimes the sheet is even charged with same sign as toner. In such a case, even repulsion occurs between toner and the sheet. And, even when a bias which is usually sufficient to prevent the offset phenomenon is applied to the rotating member, the offset phenomenon cannot be sufficiently prevented. The amount of static electricity of the sheet which is removed is easily influenced by the kind of the sheet, environments and the like. For example, under the condition of high humidity, said phenomenon is likely to happen because static electricity removal current tends to be greater.
On the contrary, the above-mentioned phenomenon (2) is likely to happen when little static electricity is removed during the separation process, which can be explained as follows. When little static electricity is removed during separation, a lot of static electricity with different sign with respect to toner remains in the sheet, which is contrary to the case of (1) described above. Accordingly, the offset phenomenon rarely occurs during the fixing process. After the fixing process, however, when a bias with same sign as toner is applied to the rotating member for fixing in order to separate the sheet therefrom, strong electrostatic attraction occurs between the sheet and the rotating member because the sheet is charged a lot with different sign with respect to toner. Therefore, separation failure is likely to happen. Such a phenomenon may happen especially when a static electricity removal needle, and the like which serves as the separation means deteriorates with age and can remove less static electricity.